OX40 is a member of the TNFR-superfamily of receptors and was first identified in 1987 as a 50 kDa glycoprotein expressed on activated CD4+ T cells from the rat (Paterson D J et al., (1987) Mol. Immunol. 24: 1281-90). The extracellular ligand binding domain of OX40 is composed of 3 full cysteine-rich domains (CRDs) and a partial, fourth C-terminal CRD (Bodmer J L et al., (2002) Trends Biochem. Sci. 27: 19-26). The ligand for OX40 is OX40L (CD252) and 3 copies of OX40 bind to the trimeric ligand to form the OX40—OX40L complex (Compaan D M & Hymowitz S G (2006) Structure, 14: 1321-1330). OX40 is a membrane-bound receptor; however a soluble isoform has also been detected (Taylor L & Schwarz H (2001) J. Immunol. Methods, 255: 67-72). Unlike CD28, OX40 is not constitutively expressed on naïve T cells but is induced after engagement of the T-Cell Receptor (TCR). OX40 is a secondary costimulatory molecule, expressed after 24 to 72 hours following activation; its ligand, OX40L, is also not expressed on resting antigen presenting cells, but is following their activation. OX40 is expressed mainly by activated CD4+ T cells and to a limited extent, by activated CD8+ T cells (Salek-Ardakani S et al., (2006) Curr. Immunol. Rev. 2: 37-53).